The Nuclear Materials Security Index notes progress in several countries. In the last two years, seven have given up most of their weapons-grade uranium and plutonium, while others are tightening security measures.

The final delivery of low enriched uranium fuel under the Megatons to Megawatts program arrives at USEC’s Paducah Gaseous Diffusion Plant in Paducah, Ky., Dec. 11, 2013. The fuel was shipped from St Petersburg, Russia in November. With this shipment, 500 metric tons of highly enriched uranium from Russian nuclear warheads, equivalent to 20,000 nuclear weapons, has now been downblended into fuel for America’s commercial nuclear power plants to use in generating electricity.

Washington — Pakistan is often depicted as an unstable nuclear power where extremists might someday get their hands on deadly nuclear materials. But in the past two years, Pakistan has addressed some gaps in nuclear materials regulations and improved its overall nuclear security.

And over the same two years, seven countries – including Ukraine, once feared as a potential source of “loose” nuclear materials that could fall into the hands of terrorists – have given up most of their weapons-grade uranium and plutonium. That reduced by almost one-fourth the number of countries in the world with appreciable amounts of fuel for atomic bombs.

That good news on global nuclear security is part of an international report card out this month that finds progress in the safekeeping of nuclear materials in much of the world. Called the Nuclear Materials Security Index, the report by the Washington-based Nuclear Threat Initiative (NTI) points to Japan, Belgium, and Canada as countries making significant improvements in the safeguarding of nuclear materials.

Japan, the report notes, made considerable strides in nuclear security after the Fukushima reactor disaster in 2011, among other things setting up a new regulatory body to take up nuclear safety and security issues.

The report is not all roses, however, and even has some critical words for the United States – which at President Obama’s initiative in 2010 launched what have become biennial summits to nudge countries to improve the security of nuclear materials.

After the first summit in Washington in 2010, the 2012 summit was held in Seoul, and in March world leaders will meet on the issue in The Hague. Mr. Obama recently announced that the US will host the 2016 summit, which is likely to be the last.

The US slipped to 11th place in this year’s index for failing to ratify two international nuclear accords it has committed to implementing, and for excluding a facility that handles nuclear materials from agreed safeguards.

In announcing this year’s results, the NTI lauded the series of summits for how they have encouraged countries to make security strides. But it also said the world must move beyond an initiative that counts on individual country action to a global security system with standards and incentives for countries to go farther.

“We are in a race between cooperation and catastrophe. We must run faster,” said former US senator and NTI co-chairman Sam Nunn, reviewing nuclear challenges and security issues before the American Nuclear Society late last year.

In issuing this year’s index, Mr. Nunn – who has championed arms-control and nuclear nonproliferation issues for decades – noted that “a nuclear detonation in any part of the world will affect us all.” He called for leaders at the summit in The Hague to set the foundations of a global security system. “We need a global nuclear materials system to secure all materials, to employ international standards and best practices, and to give states the capacity to assess nuclear security globally and hold each other accountable,” he added.

Still, the 2014 index, which NTI developed in conjunction with the Economist Intelligence Unit, a London-based risk assessment organization, suggests the current system is moving the world forward.

In addition to Ukraine, the other countries that rid themselves of most of their holdings of uranium and plutonium were Austria, the Czech Republic, Hungary, Mexico, Sweden, and Vietnam. That reduced the number of countries with appreciable amounts of those fuels from 32 to 25. The index also ranks 151 other countries that have little or no weapons-grade materials but which could be used as safe havens or transit points for what are often called “loose nukes.”

The drop in countries with bomb-fueling materials does not mean that no extremely worrisome gaps in nuclear security remain, as NTI points out. North Korea remains at the bottom of the list, with the index finding the pariah state consistently deficient on most measures of nuclear security. Also at the bottom of the list are Iran, India – and Pakistan, despite the improvements it registered.

Moreover, concerns about the potential for terrorists to get their hands on materials to make a “dirty bomb” extend beyond uranium and plutonium alone.

In December two men were arrested in the Caucasus country of Georgia for attempting to traffic a quantity of the radioactive material radium 266 sufficient to build a dirty bomb. And in December in Mexico, alarm bells heard as far away as the International Atomic Energy Agency in Vienna were rung after thieves stole a truck carrying a small but extremely dangerous quantity of the radioactive isotope cobalt-60. The truck and the material – enough if ground down to fuel a dirty bomb, according to nuclear experts – were later recovered.

The two incidents were reminders of what small amounts of nuclear materials are required to build weapons – and why the NTI says “much remains to be done” to secure the estimated 2,000 metric tons of the major weapons-usable materials – highly enriched uranium or plutonium – in the world.

“All it takes to build a bomb is enough highly enriched uranium to fill a five-pound bag of sugar,” the report notes, “or a quantity of plutonium the size of a grapefruit.”